1. Field of the Invention
The present invention relates to a vehicular power control apparatus to perform power control of a hybrid vehicle which uses an internal combustion engine and an AC motor as sources of power, and particularly to a vehicular power control apparatus which detects an abnormality due to a short circuit fault of a component part of an electric power converter for converting DC power into AC power and supplying it to an AC motor, and performs power control in accordance with the abnormality.
2. Description of the Related Art
Conventionally, there is used a power control apparatus for a hybrid vehicle, which includes an internal combustion engine, its control apparatus, an AC motor, an electric power converter to supply AC power to the AC motor and its control apparatus, and combines both the output of the internal combustion engine and the output of the AC motor to use them as the sources of power of the vehicle. The electric power converter is an apparatus to mutually convert DC power and AC power, and at the time when the AC motor is driven, the DC power supplied from the DC power supply is converted into the AC power and is supplied to the AC motor, and the AC motor is driven. Besides, at the time of braking of the AC motor, the AC power generated by the AC motor is converted into the DC power and is supplied to the DC power supply, and the AC motor is made to perform a regenerative operation.
The electric power converter incorporates, as a switching unit, a semiconductor power element (hereinafter referred to as a power element) including a transistor such as, for example, an IGBT (Insulated Gate Bipolar Transistor) and a diode, and is constructed to perform electric power conversion by the conducting (ON) and nonconducting (OFF) operation of the power element caused by switching signals outputted from the control apparatus.
Incidentally, it is of course that the power element as the switching unit can be made of a semiconductor element other than the IGBT.
The control of the output of the AC motor used in the hybrid vehicle is generally performed by controlling its output torque quantity. The output torque quantity is changed in correlation with the amount of current flowing to the AC motor, and as the amount of current becomes large, the amount of output torque is increased. From this, the output control of the AC motor is performed by controlling the amount of the flowing current. Since the current flowing through the AC motor flows also to the power element, in the case where the amount of current is large, that is, in the case where the amount of output torque is large, the amount of heat generation is also large by the resistance component of the power element, and the degree of temperature rise of the power element becomes large.
Besides, as the characteristic of the power element, there is a case where the gate applied voltage by a switching signal given to the gate portion does not reach a suitable standard, and the switching element performs the conducting operation in an unsaturated region. In this case, since the resistance component of the power element becomes large, the amount of heat generation becomes large even for the same amount of current, and the temperature of the power element rises.
In the case where the IGBT is used as the power element, when the temperature of the IGBT rises, the amount of current which can be interrupted at the time of the nonconducting operation becomes low, and it is damaged by the occurrence of latch-up. Thus, for the purpose of protecting the power element, a protection operation is often used in which in the case where a large current not lower than a specified value flows (ON; Over Current), or in the case where the temperature rises up to a specified value or higher (OT; Over Temperature), or in the case where the gate applied voltage at the time of the conducting (ON) operation becomes a specified value or lower (UV; Under Voltage), even in the case where a conducting (ON) operation instruction is given from outside, this instruction is made invalid to forcibly interrupt the switching signal and to make the power element nonconducting (OFF).
The protection operation by the interruption of the switching signal is continuously performed over a specified period of about 10 [ms], and after the specified period is ended, the interruption of the switching signal is stopped and the protection operation is released. Besides, the state that the protection operation is being performed is notified to a control arithmetic unit as a generation source of the switching signal by a protection operation identifying signal (FO; Fault Output). The control arithmetic unit judges whether the motor operation should be continued or stopped according to the recognition of the protection operation identifying signal FO or the recognition of intermittent occurrence of the continuous protection identifying signal FO in the short time, and in the case where a judgment is made that the operation should be stopped, the switching signal is stopped, and an operation is perform to keep the nonconducting state.
That is, the conventional power control apparatus for the hybrid vehicle as stated above stops the operation of the AC motor by stopping the switching of the power element of the electric power converter, and releases the abnormal state and avoids the dangerous state of the vehicle.
However, in the case where the power element is subjected to a short circuit fault, even if the interruption of the switching signal, that is, the protection operation by the gate interruption, or the keeping of the nonconducting (OFF) state by the stop of the switching signal from the control arithmetic unit is performed, a large current continues to flow according to the rotation state of the AC motor, and the abnormal operation of the AC motor can not be stopped. This occurs because the balance of voltages in respective phases applied to lines of the AC motor is lost since the terminal voltage of the AC motor becomes equal to the potential of the electric power converter at the DC side by the short circuit fault of the power element.
Besides, in a permanent magnet type AC motor in which a permanent magnet is used as a rotator magnetic pole to generate a magnetic flux, even if the switching of the power element is stopped, an induced voltage proportional to the rotation speed of the rotator magnetic pole is generated in an armature winding and it operates as a generator. Thus, when the rotation speed of the AC motor is kept to be high, a large current continues to flow to an electric power supply passage.
The permanent magnet type AC motor often used for the hybrid vehicle is designed so that in addition to the request for miniaturization and reduction in weight, the amount of magnetic flux of the permanent magnet relatively becomes large with respect to a volume, and therefore, there is a tendency that the ratio of the induced voltage to the rotation speed becomes high. Thus, in the case where the rotation speed rises and exceeds the upper limit in which the electric power converter can control the output, the so-called flux-weakening control is performed in which the phase of an applied voltage with respect to the rotation angle of the rotator is adjusted, and the increase of a motor terminal voltage is suppressed.
Here, in the case where the nonconducting (OFF) state of the power element is kept for protection against occurrence of some abnormality, the flux-weakening control of the AC motor is not performed, and therefore, a high voltage exceeding the rating is applied to the electric power converter and the AC motor, and further, in the case where the power element is subjected to a short circuit fault, a large current exceeding the rating continues to flow to the electric power converter and the AC motor. Thus, a large current exceeding the voltage resistance and heat resistance of component parts forming the electric power supply passage, such as the power element, a connection terminal between the electric power converter and the AC motor, and the armature winding of the AC motor, and there is a fear of the occurrence of possibility that a secondary fault such as smoking, burnout or dielectric breakdown occurs. The occurrence of the smoking, burnout or dielectric breakdown is an important problem relating to the safety of the vehicle, and it is necessary to prevent those from occurring.
Then, conventionally, in order to cope with such a problem, it is proposed that a fusible part functioning as a fuse is provided as an electrode member to connect a semiconductor portion of a power element and an external current passage, and when a current of a specified value or higher flows, the fusible part is fused to cut the circuit (for example, see patent document 1 (JP-A-2005-175439, pages 2 to 12, FIGS. 1 to 14)).
Besides, in this related art, it is exemplified that the fusible part is constructed of a bonding wire having a structure different from the electrode member.
However, even in the conventional vehicular power control apparatus using the power element disclosed in patent document 1, there are following problems:
(1) Since the principle that the fusible part of the power element is fused is based on melting by heat generation from the resistance component of the fusible part, as compared with a power element having no fusible part, the resistance value of the fusible part is high, and therefore, an unnecessary loss always occurs, and the balance to cooling performance and characteristics relating to efficiency are deteriorated.
(2) The fusible part is inserted in series to the electric power supply passage, and in the case where the fusible part is erroneously fused, the vehicular power control apparatus does not normally operate, and the reliability of the apparatus is lowered.
(3) When the fusing current value is set to be low so that the fusible part is certainly fused at the time of the short circuit fault of the power element, the fusing current set value can be smaller than the permissible current of the semiconductor portion of the power element, and therefore, the rated current capacity in the whole power element is lowered. On the other hand, when the fusing current value of the fusible part is set to be high, even if the power element is already subjected to the short circuit fault and an abnormal current flows, it becomes impossible to certainly fuse the part.
(4) In the case where the fusible part of the power element is constructed of the bonding wire, and the power element is molded with an injection molding resin for insulation, poor contact occurs in the bonding part, or a contact portion between the cut place of the bonding wire body and another portion, and there is a possibility that the conduction of current becomes intermittent. In such a case, an arc by a high voltage occurs at the poor contact place, and there is a possibility that the mold member smokes or fires.
The above problem (4) in the combination of the bonding wire and the insulating injection mold resin mold is also the worrying problem in the structure of the semiconductor element body. Conventionally, there is provided a power element in which an electrode of a semiconductor element and a metal plate positioned around the semiconductor element and to form a current flow passage are electrically connected to each other by many bonding wires, and these are integrally molded with resin to be insulated and are fixed. Although the power element constructed in this way effectively contributes to the improvement of performance relating to vibration resistance, temperature change resistance characteristic, heat radiation characteristic and the like, the problem as recited in the above (4) can arise. Besides, when the short circuit fault occurs, the resistance value at the fault place is lowered, uniform current does not flow to the respective bonding wires, and the current is concentrated on the wire around the fault place and unevenly flows. As stated above, when the current is concentrated on the place of the short circuit fault and flows, the possibility of the smoking and firing becomes high.
On the other hand, because of the restriction of a process time and process area of a bonding part in the case where a bonding wire is connected to a semiconductor by bonding, in order to realize miniaturization and reduction in cost of a power element, a direct lead bonding (DLB) system is proposed in which instead of a wire, a metal plate is connected to a semiconductor to achieve electrical connection. In this direct lead bonding system, the unevenness of the current as in the wire bonding system hardly occurs, and an arc due to poor contact of a connection part also does not occur, and therefore, the possibility of the smoking and firing is suppressed to be low. However, even in the power element of the direct lead bonding system, there is an unsolved problem that when the motor rotation speed is kept to be high at the time of a short circuit fault, a large current continuously flows to the electric power supply passage.